Transparent universe reveals hidden galaxies

By Rachel Courtland

THE universe is far more transparent at high energies than we thought. This discovery – based on sightings of unexpectedly bright objects that should be too far away to see so clearly – may call into question our understanding of how galaxies are born and evolve.

The universe is more transparent than expected, questioning what we know of galaxy formation

Most light travels through the cosmos unimpeded. But photons with very high energies of more than 100 gigaelectronvolts can collide with intergalactic infrared light. The longer these photons have to travel, the greater their chances of colliding and the less likely they are to reach Earth. As a result, distant blazars – galaxies with gluttonous black holes at their centres whose flares are pointing directly at Earth – are supposed to be much dimmer at higher energies than those that are not so far off.

Based on estimates of the amount of infrared light pervading the universe, blazars more than a billion years old were expected to be mostly invisible to telescopes looking for very high-energy gamma rays, says astrophysicist Simon Swordy of the University of Chicago.

But in 2006, the HESS telescope in Namibia reported the discovery of two unexpectedly bright blazars that are more than 2 billion years old. What’s more, bright light from a blazar called 3C279, spotted one night in 2007 by the MAGIC telescope on La Palma in Spain’s Canary Islands, survived some 5 billion years of travel. “We can see significantly further than we thought we could,” says Swordy.